Mike-

The key to your search it to always be set to jump, regardless of your state of preparedness!
I've gone into ultra scramble mode a couple of times for the chance to get such nearly impossible to find treasures.
Next, look where the action is.

I keep my eyes on a few websites --
-Gunsinternational (I check for new items every day here!)
-Gunbroker
-Auctionarms
-Champlin Arms
-Monty Whitley
-Lewis Drake
-Henry Krank
-Mick Shepherd
-Holt's Auction
-Julia Auction
-Greg Martin Auction
-(there are others...)

I also do frequent Google searches using quotation marks around creative and imaginative ways to say what it is that I'm looking for.
Stuff comes up in the strangest places.

Also I keep myself in conversation with a few gentlemen who live all over the globe who know my weaknesses, and we lend support to each other through the seasons via telephone and email.
It's good to stay in touch with each other anyway, and while in the chase for things that suit their needs I've found some interesting things for myself too.
(thanks to those of you who are reading..!)

I'll get a couple of sixteen bore rifles out later on and measure the wall at muzzle, section mid way, and section at chamber throat.
More than anything, that kind of thing is about balance.
You'd be surprised at how little metal is needed to contain bore-rifle pressures - the bigger the bore, the less pressure you'll need to get the beloved (smack!) impact energies.
For a take on 16bore VS 12bore, my Mahillon 2-barrel set is a 16bore double rifle with a second set of 12bore shotgun barrels. It's built on a 12bore frame.

Also, be prepared to get second opinions on what you're looking at.
Emotion can blur your better judgment when something neat finally comes up - but be straight with yourself on whether or not you can properly evaluate antique double rifles.
Also know where the line-in-the-sand lies when it comes to condition. High grade firearms can be a relatively safe place to put your money, they can also be an awfully easy place to lose your money if you don't have relative market comparisons or a solid sense of condition. Value can be subjective when you're shopping for a toy, but you should consider what may come up when/if the time to sell comes around...

Cheers
Tinker